Rubber-modified edge coating for honeycomb used in panels with composite face sheets

ABSTRACT

Core-skin bonding is improved between honeycomb and composite face sheets by applying a polyamide and/or rubber-containing adhesive to the edge of the honeycomb prior to bonding. Edge coating of honeycomb with a polyamide and/or rubber-containing adhesive is useful in further increasing the bond strength between honeycomb and prepreg face sheets, especially when the matrix resin of the prepreg is designed to adhere the prepreg to the honeycomb without the use of a separate structural adhesive.

This is a continuation-in-part of co-pending U.S. patent applicationSer. No. 11/295,829 filed Dec. 7, 2005, which is a continuation-in-partof co-pending U.S. patent application Ser. No. 10/932,510 filed Sep. 1,2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to sandwich panels and otherrelated structural composite materials. Sandwich panels are typicallymade up of face sheets (also referred to as “skins”) that are adhesivelybonded to opposite sides of a core material, such as honeycomb, to formthe sandwich panel. In particular, the present invention relates totreating honeycomb in order to improve the bond between the honeycomband face sheets. The invention is particularly applicable to sandwichpanels where the face sheets are made from composite materials.

2. Description of Related Art

Sandwich panels are used in a wide variety of applications where highstrength and light weight are required. The cores that are used in mostsandwich panels are either lightweight honeycomb, rigid foam, paper orwood. Honeycomb is a popular core material because of its high strengthto weight ratio and resistance to fatigue failures. A wide variety ofproducts including metals and composite materials have been used to makethe honeycomb cores.

The face sheets that are bonded to the side (also referred to as the“edge”) of the honeycomb have also been made from a wide variety ofmaterials including metals and composites. An important consideration inthe formation of any sandwich panel is the way in which the face sheetsare bonded to the honeycomb. Typically, an adhesive is used to bond theface sheets to the core. The adhesive must rigidly attach the facings orskins to the core in order for loads to be transmitted from one facingto the other and to permit the structure to fulfill all the assumptionsimplied in the acceptance of the commonly used stress calculationmethods. If the adhesive fails, the strength of the panel is severelycompromised. The adhesive is especially critical in sandwich panelswhich use honeycomb as the core because of the relatively small surfacearea over which the edges of the honeycomb contact the face sheets.

One procedure for applying composite face sheets to honeycomb involvesforming a prepreg sheet that includes at least one fibrous reinforcementlayer and an uncured resin matrix. Prepreg is a term of art used in thecomposite materials industry to identify mat, fabric, non-wovenmaterial, tow or roving which has been pre-impregnated with resin andwhich is ready for final curing. A film adhesive is typically added tothe prepreg-core assembly and it is then bonded to the honeycomb bycuring of both the prepreg resin and adhesive resin at elevatedtemperature. The film adhesive can be applied as a separate ply layer oras an integral part of the prepreg sheet.

Honeycomb sandwich panels are used in many applications where stiffnessand structural strength of the panel are primary considerations.Additionally, honeycomb sandwich panels are also widely used in theaerospace industry where the weight of the panel is also of primaryimportance. As a result, there has been and continues to be a concertedeffort to reduce the weight of the honeycomb sandwich panels withoutsacrificing structural strength. One area which has been investigated toreduce weight is the elimination of separate adhesive layers. This hasbeen accomplished by making the face sheets from composite materialsthat are self-adhesive. The resins used in such self-adhesive prepregsmust meet the dual requirements of providing suitable structuralstrength while still providing adequate adhesion to the honeycomb.Exemplary self-adhesive face sheets are described in published EuropeanPatent Application Nos. EP0927737 A1 and EP0819723 A1 and in U.S. Pat.Nos. 6,440,257 and 6,508,910.

An alternative method of bonding face sheets to honeycomb involvesapplying an adhesive to the edge of the honeycomb. The adhesive istypically applied by “dipping” the edge of the honeycomb in theadhesive. The adhesives used in this type bonding are typically referredto as “dip” resins or adhesives. The advantage of this method is thatthe adhesive is located only where the honeycomb contacts the facesheet, rather than being distributed over the entire face sheet. Thismethod is generally used to bond non-adhesive face sheets, such asaluminum and other metallic face sheets, to the honeycomb.

SUMMARY OF THE INVENTION

In accordance with the present invention, honeycomb sandwich panels areprovided that include at least one face sheet that has an interiorsurface that is bonded to a honeycomb core and an exterior surface. Theface sheet includes at least one fiber layer and a resin matrix. Thehoneycomb includes walls that define a plurality of honeycomb cellswherein the walls have at least one edge that is bonded to the interiorsurface of the face sheet. As a feature of the invention, an adhesivethat includes polyamide (a common example of which is commonly referredto as “nylon”) and/or rubber is used to bond the face sheet to thehoneycomb. It was discovered that the use of an adhesive that includespolyamide and/or rubber provides an especially strong bond between thehoneycomb and face sheets where the face sheet includes a resin matrix.The use of a polyamide and/or rubber-containing adhesive in accordancewith the present invention was found to be particularly useful inincreasing the bonding strength between honeycomb and prepreg facesheets.

As a further feature of the present invention, adhesives that includepolyamide and/or rubber provide an increase in the bond strength betweenhoneycomb and prepreg face sheets where the resin matrix used for theprepreg includes a thermosetting resin. The use of polyamide and/orrubber-containing adhesives is particularly useful in increasing thebond strength of self-adhesive prepreg face sheets where the prepregresin matrix includes a rubber and/or thermoplastic toughenedthermosetting resin. In some embodiments of the invention, the bondstrength provided by the polyamide and/or rubber-containing adhesive issuch that, during separation of the face sheet from the core (in drumpeel tests), the honeycomb wall is torn apart while the bond between theface sheet and the honeycomb remains intact. In these embodiments, theadhesive bond is stronger than the core so that the sandwich panelundergoes core failure rather than cohesive failure when the face sheetis peeled from the panel.

As an additional feature of the present invention, adhesives thatinclude polyamide and/or rubber are particularly well suited for bondingface sheets to honeycomb that is made from composite materials. Duringthe fabrication process, a honeycomb slice is typically cut from a largeblock of honeycomb. The slicing of the honeycomb from the block tends toexpose fibers and form “ragged” or “fuzzy” edges that tend to bedifficult to bond to the face sheet. Accordingly, in order to improvebond strength, the edges of composite honeycomb are generally treatedprior to bonding by sanding, machining or other surface treatment toremove the exposed fibers and smooth the honeycomb edge. The polyamideand/or rubber-containing adhesives in accordance with the presentinvention provide good bond strength when either fuzzy or smoothhoneycomb is used. Accordingly, the time consuming step of removing thefuzzy edges from the composite honeycomb may be deleted, if desired.

The present invention not only covers sandwich panels as described abovethat are made using honeycomb which is edge-coated with a polyamideand/or rubber-containing adhesive, but also covers methods for bondingface sheets to honeycomb to form such sandwich panels. The methodsinvolve providing a face sheet that includes an interior surface forbonding to the honeycomb core and an exterior surface wherein the facesheet includes at least one fiber layer and a resin matrix. A honeycombis provided that includes walls that define a plurality of honeycombcells wherein the walls have at least one edge for bonding to theinterior surface of the face sheet. An adhesive that includes polyamideand/or rubber is applied to the honeycomb edge to provide an edge-coatedhoneycomb that is then bonded to the face sheet.

The finished sandwich panels made in accordance with the presentinvention may be used in a wide variety of situations where alightweight and structurally strong material is needed. However, theinvention is especially well suited for use in aerospace applicationswhere a multitude of strict mechanical and chemical requirements must bemet while at the same time not exceeding weight limitations. Theabove-described and many other features and attendant advantages of thepresent invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred exemplary honeycomb core andtwo composite face sheets prior to bonding of the honeycomb to the facesheets using an edge coating adhesive in accordance with the presentinvention that contains polyamide or a modified polyamide and/or rubber.

FIG. 2 is a perspective view of a preferred exemplary sandwich panelwhere the components shown in FIG. 1 have been bonded together and curedto form the final honeycomb sandwich panel.

FIG. 3 is a partially diagrammatic representation of an exemplary methodin accordance with the present invention showing a single edge-coatedhoneycomb wall being bonded to a face sheet.

FIG. 4 is a partially diagrammatic view showing an embodiment of thepresent invention where the sandwich panel undergoes core failure duringforcible separation of the face sheet from the honeycomb.

DETAILED DESCRIPTION OF THE INVENTION

It is preferred that the nylon (polyamide) and/or rubber-containingadhesives in accordance with the present invention be used to improvethe bonding of composite face sheets (particularly self-adhesiveprepregs) to honeycomb cores to form light-weight structural panels foruse in aerospace applications where light weight and structural strengthare especially important design criteria. However, those of ordinaryskill will recognize that the present invention is not limited toaerospace applications, but may also be used in any situation wherethere is a need for honeycomb sandwich panels that have face sheetswhich are made from composite materials.

The three basic components of a preferred exemplary honeycomb sandwichpanel for use in aerospace applications are shown in FIG. 1 prior toformation of the panel. The components include a honeycomb core 12 thathas walls 11 which form a plurality of honeycomb cells 13. The wallshave edges that form the faces or edges of the honeycomb as shown at 14and 16. The other two components are prepreg face sheets 17 and 19. Theface sheets 17 and 19 include interior surfaces 21 and 23, respectively,for bonding to the honeycomb edges. The face sheets 17 and 19 alsoinclude exterior surfaces 25 and 27, respectively. In accordance withthe present invention, the edges 14 and 16 of honeycomb 12 are coatedwith an adhesive (not visible in FIG. 1) that includes polyamide and/orrubber as an essential ingredient to form an edge-coated honeycomb. Theuse of an adhesive that contains polyamide and/or rubber in accordancewith the present invention improves the bonding of face sheets 17 and 19to the honeycomb 12, while adding very little to the weight of theresultant panel. The uncured prepreg face sheets or skins 17 and 19 areapplied to the edge-coated honeycomb 12 and then cured to form facesheets 18 and 20 of the finished panel 10 as shown in FIG. 2.

FIG. 3 is a partially diagrammatic representation showing the method forbonding the honeycomb to one of the face sheets 19 in accordance withthe present invention. For simplicity, FIG. 3 shows the bonding of asingle wall 111 with it being understood that the entire edge 16 of thehoneycomb is coated with the polyamide and/or rubber-containing adhesive(coating). As shown in FIG. 3, the lower edge of honeycomb wall 11 iscoated with a polyamide and/or rubber-based adhesive 15 prior to bondingto the lower uncured prepreg face sheet 19. As represented by arrow 30,the edge-coated honeycomb is located against the face sheet 19 such thatthe polyamide and/or rubber-containing adhesive flows partially onto theprepreg 19 to form fillets 28 and 29. Then, as represented by arrow 40,the prepreg is cured to form the final sandwich panel 10 that includesthe cured face sheet 20.

The honeycomb core 12 can be made from any of the materials that areused to form honeycomb cores. Exemplary honeycomb materials includealuminum, aramid, carbon or glass fiber composite materials, resinimpregnated papers, non-woven spun bonded polypropylene, spun bondednylon, spun bonded polyethyleneterephthlate (PET), and the like.Exemplary preferred honeycomb materials are aramid-based substrates,such as those marketed under the trade name NOMEX® which are availablefrom E.I. DuPont de Nemours & Company (Wilmington, Del.). Honeycombcores made from NOMEX® are available commercially from HexcelCorporation (Dublin, Calif.). Preferred exemplary NOMEX® honeycombinclude HRH®10 which is available from Hexcel Corporation. Anotherpreferred honeycomb material is KEVLAR®. Preferred exemplary KEVLAR®honeycomb is available from Hexcel Corporation under the trade nameHRH®36. Honeycomb made from carbon or glass composites are alsopreferred and typically include carbon or glass fabric and a phenolicand/or polyimide matrix. The honeycomb is typically supplied in a curedform and requires no further treatment prior to bonding to the skins orface sheets other than application of the polyamide-based adhesive asdescribed below. If desired, the cores may be sanded or otherwisemachined to remove the “fuzzy” edges that may be present due to fibersthat are exposed during slicing of the honeycomb from the larger blockof honeycomb. However, as mentioned previously, the use of polyamideand/or rubber-containing adhesives in accordance with the presentinvention as an edge coating eliminates or at least substantiallyreduces the need for removing the fibers that are exposed on thehoneycomb edges during fabrication.

The dimensions of the honeycomb can be varied widely. For aerospace use,the honeycomb cores will typically have ⅛ to ½ inch (3.2-12.7 mm) cells(i.e., in expanded direction) with the cores being ¼ inch (6.4 mm) to 2inches (50.8 mm) thick (distance between the honeycomb edges). Thethickness of the honeycomb walls may also be varied with typicalhoneycomb walls being on the order of 0.001 inch (0.25 mm) to 0.005 inch(0.13 mm) thick. The combination of cell size, wall thickness anddensity of the material that is used determines the weight of the corewhich is expressed in pounds per cubic foot (pcf). Composite honeycombhaving weights on the order of 2 pcf to 8 pcf are preferred.

The face sheets that are bonded to the honeycomb can be made from any ofthe materials that are used in composite sandwich panel construction.Such composite materials usually include one or more layers of fibersand a resin matrix. The fibers that are used in the prepreg face sheets17 and 19 can be any of the fiber materials that are used to formcomposite laminates. Exemplary fiber materials include glass, aramid,carbon, ceramic and hybrids thereof. The fibers may be woven,unidirectional or in the form of random fiber mat. Woven carbon fibersare preferred, such as plain, harness satin, twill and basket weavestyles that have areal weights from 80-600 gsm, but more preferably from190-300 gsm. The carbon fibers can have from 3,000-40,000 filaments pertow, but more preferably 3,000-12,000 filaments per tow. All of whichare commercially available. Similar styles of glass fabric may also beused with the most common being 7781 at 303 gsm and 120 at 107 gsm. Whenunidirectional constructions are used, typical ply-weights are 150 gsmfor carbon and 250 gsm for glass.

Although the composite material may be pre-cured prior to bonding to thehoneycomb, it is preferred that resin matrix be in an uncured orpartially cured state prior to bonding. The use of uncured face sheetsis preferred because it combines curing and bonding of the face sheet tothe honeycomb into a single step and allows the polyamide and/orrubber-containing adhesive present on the honeycomb to interact with theuncured resin matrix during the bonding process. Prepreg is a preferredform or type of uncured face sheet.

The resin matrix for the prepreg should include a thermosetting resinsuch as epoxy, cyanate ester, phenolic, benzoxazines, polyimide orbismaleimide. Prepregs that have some inherent adhesiveness(self-adhesive prepreg) are particularly preferred. Prepregs of the typedescribed in issued U.S. Pat. Nos. 6,440,257 and 6,508,910 are examplesof self-adhesive prepregs that are capable of bonding to honeycomb toform a suitable sandwich panels without the use of an adhesive layer.These prepregs include a resin matrix that is a combination ofthermosetting and thermoplastic resins. Use of nylon and/or rubber-basedadhesives to coat the honeycomb edges was found to provide an unexpectedincrease in bond strength between the honeycomb and these types ofself-adhesive face sheets.

The preferred resins that are used to form the resin matrix includeepoxy and/or cyanate ester and/or bismalemide resins and one or morecuring agents. The resin matrix also preferably includes viscositycontrol agents and thermoplastic fillet forming particles as describedin U.S. Pat. Nos. 6,440,257 and 6,508,910. When viscosity control agentsand fillet forming particles are included in the resin matrix, theresin(s) are first mixed with viscosity control agents to form a resinmixture. If necessary, the mixture is heated to ensure that viscositycontrol agents are completely dissolved. Curing agents and filletforming particles are then added to the resin mixture. This final resinmixture is kept below the temperature at which the fillet formingparticles dissolve in the resin. As a result, the fillet formingparticles, which at this stage are preferably uniformly mixed throughoutthe resin, are not dissolved to a substantial degree and therefore donot increase the resin viscosity to an unacceptable level. The viscosityof the resin mixture is important because it must be such that the resincan be impregnated into the fiber to form the prepreg. For the purposesof this specification, particles which retain at least 90 weight percentof their original particle weight are considered to be not dissolved toa substantial degree. Particles are considered to be substantiallydissolved when less than 10 percent by weight of the original particleremains intact within the resin.

The viscosity of the final resin mixture that is used to make theprepreg, including fillet-forming particles, should be between 150 and1500 poise. The preferred viscosity is between 300 to 1200 poise. Thepreceding viscosity ranges represent minimum viscosities for the finalresin mixture prior to making prepreg when said viscosity is measured byRheometric Dynamic Analysis (Rheometrics RDA2) at settings of 2° C./min,10 rads/sec and 0.8-1.0 mm gap. The viscosity of the resin mixturegradually increases when the fillet forming particles dissolve duringthe curing/bonding process.

During fabrication of the prepreg process the fillet forming particlestend to be concentrated toward the surface of the prepreg due toinherent filtering of the particles by the fiber layer. Alternatively,the fillet forming particles may be applied (e.g., by powder deposition)to the surface of the resin after it has been formed into a prepreg filmor after the resin has been impregnated into the fiber layer. In thisway, the fillet forming particles are distributed substantially on thesurface of the prepreg. In either case, the resin temperature ismaintained at a sufficiently low level to prevent the fillet formingparticles from dissolving until the prepreg is applied to the corematerial and cured.

After application to the edge-coated core, the prepreg is heated to asufficient level for a sufficient time to substantially dissolve thefillet forming particles, cure the prepreg and cure the polyamide-basedadhesive. The dissolved thermoplastic particles enhance the toughness ofthe bond and interact with the polyamide adhesive (as well as thethermosetting resin present in the prepreg matrix resin) to even furtherstrengthen the bond.

Exemplary thermosetting resins which may be used to make the prepregmatrix resin include phenolics, benzoxazines, polyimides, epoxy, cyanateester and bismaleimide resins. Exemplary epoxy and cyanate ester resinsinclude glycidylamine type epoxy resins, such astriglycidyl-p-aminophenol, tetraglycidyldiaminodiphenyl-methane;glycidyl ether type epoxy resins, such as bisphenol A type epoxy resins,bisphenol F type epoxy resins, bisphenol S type epoxy resins, phenolnovolac type epoxy resins, cresol novolac type epoxy resins (e.g. ECN1299 available from Huntsman Advanced Materials, Inc., Brewster, N.Y.)and resorcinol type epoxy resins; cyanate esters, such as1,1′-bis(4-cyanatophenyl) ethane (AroCy L-10, available from HuntsmanAdvanced Materials, Inc., Brewster, N.Y.), 1,3-Bis(4-cyanateophenyl-1-1-(1-methylethylidene) benzene (RTX366, availablefrom Huntsman Advanced Materials, Inc., Brewster, N.Y.). Epoxy resinsare preferred. Especially preferred epoxy blends include a mixture oftrifunctional epoxy and a difunctional bis-F epoxy.

Curing agents and viscosity control agents (optional) are also added tothe thermosetting resin to form the basic prepreg matrix resin. Thecuring agent is preferably an amine curing agent and the viscositycontrol agent is preferably a thermoplastic material that dissolves inthe thermosetting resin.

Although a wide variety of matrix resins may be used in the compositeface sheets, matrix resins based on epoxy and cyanate ester formulationsare preferred. In addition, sandwich panels that have two prepreg plieson each face of the honeycomb are preferred. The two layers arepreferably either two (0/90) plies or two (±45, 0/90) plies with warpdirection aligned with the lengthwise direction of the honeycomb. Thoseof ordinary skill in the art will recognize that the present inventionis also applicable to multiple ply face sheets that include two or morefiber layers.

Exemplary preferred face sheet matrix resin formulations are as follows:

1)

-   -   1 to 70 parts by weight of an epoxy;    -   5 to 40 parts by weight of an amine curing agent;    -   1 to 30 parts by weight of a viscosity control agent; and    -   5 to 50 parts by weight of thermoplastic fillet forming        particles.

2)

-   -   10 to 40 parts by weight of a trifunctional epoxy resin;    -   10 to 40 parts by weight of a difunctional epoxy resin;    -   11 to 25 parts by weight of an aromatic curing agent;    -   0 to 3 parts by weight of a non-aromatic curing agent;    -   5 to 15 parts by weight of a viscosity control agent; and    -   8 to 30 parts by weight of thermoplastic fillet forming        particles.

The epoxy may be composed of trifunctional epoxy, difunctional epoxy anda wide variety of combinations of trifunctional and difunctionalepoxies. Tetrafunctional epoxies may also be used. Exemplarytrifunctional epoxy include triglycidyl p-aminophenol andN,N-Diglycidyl-4-glycidyloxyaniline (MY-0510 or MY-0500 available fromHuntsman Advanced Materials, Inc., Brewster, N.Y.). Exemplarydifunctional epoxies which may be used in the resin include Bis-Fepoxies, such as GY-281, LY-9703 and GY-285 which are available fromHuntsman Advanced Materials, Inc., Brewster, N.Y.). Bis-A epoxies, suchas GY-6010 (Huntsman Advanced Materials, Inc., Brewster, N.Y.) and DER331 (Dow Chemical, Midland, Mich.) are suitable Bisphenol-A type epoxiesand may also be used. An exemplary tetrafunctional epoxy istetraglycidyl diaminodiphenyl methane (MY-721, MY-720 and MY-9512available from Huntsman Advanced Materials, Inc., Brewster, N.Y.). Othersuitable epoxies include phenol novolak type epoxy, cresol novolak epoxyand resorcinol type epoxy. Preferred bis-F epoxies include GY281 andGY285 which are available from Huntsman Advanced Materials, Inc.,Brewster, N.Y.

Exemplary curative agents include dicyandiamide,3,3-diaminodiphenylsulfone (3,3-DDS), amino or glycidyl-silanes such as3-amino propyltriethoxysilane, CuAcAc/Nonylphenol (1/0.1),4,4′-diaminodiphenylsulfone (4,4′-DDS),4,4′-methylenebis(2-isopropyl-6-methylaniline), e.g., Lonzacure M-MIPA(Lonza Corporation, Fair Lawn, N.J.),4,4′-methylenebis(2,6-diisopropylaniline), e.g., Lonzacure M-DIPA (LonzaCorp., Fair Lawn, N.J.). Dicyandiamide and 3,3-DDS are preferredcurative agents. Especially preferred are combinations of 3,3-DDS anddicyandiamide.

Exemplary viscosity control agents include thermoplastic polyetherimidessuch as ULTEM® 1000P which is available from General Electric(Pittsfield, Mass.); micronized polyethersulfone such as 5003P, which isavailable from Sumitomo Chemical Co., Ltd. (Osaka, Japan); HR1-1, whichis available from Hexcel Corp. (Dublin, Calif.); and polyimide MATRIMID®9725, which is available from Huntsman Advanced Materials, Inc.(Brewster, N.Y.). ULTEM® 1000P and micronized PES are preferred.Micronized PES is especially preferred. The amount and type of viscositycontrol agent which is added to the epoxy resin mixture may be variedprovided that the minimum viscosity of the final resin mixture ismaintained between 150 and 1500 poise when said viscosity is measured byRheometric Dynamic Analysis (Rheometrics RDA2) at settings of 2° C./mni,10 rads/sec and 0.8-1.0 mm gap. As previously mentioned, mixtures withminimum viscosities of between 300 to 1200 poise are preferred. Theviscosity of the prepreg resin prior to addition of the fillet formingparticles should be between about 50 poise and 2000 poise at roomtemperature. The preferred viscosity range is 100 poise to 1500 poise atroom temperature.

Densified polyethersulfone (PES) and densified polyetherimide particlesmay be used as suitable fillet forming particles. Densified PESparticles are preferred. The densified polyethersulfone (PES) particlesare preferably made in accordance with the teachings of U.S. Pat. No.4,945,154, the contents of which is hereby incorporated by reference.The average particle size of the PES particles range from 1 to 150microns. Average particle sizes of 1 to 50 microns are preferred andaverage particle sizes of 10 to 25 microns are particularly preferred.The microspheres are generally spherical in shape and are classified bypassing the densified microsphere powder through a micron sieve. It ispreferred that the glass transition temperature (Tg) for the particlesbe above 200° C.

The PES which is used may be “micronized”, if desired. Micronized PESrefers to PES particles that have a rough surface configuration that isproduced by grinding the particles or using other abrasive techniques ofmanufacture that are known in the art. Micronized PES particles may alsobe made by using spraying and drying procedures that are also known inthe art. Micronized PES particles are preferably less than 120 micronsin size. Especially preferred are particles under 50 microns in sizewith a range of 10 to 25 microns being particularly preferred.

The prepreg resin is made by first mixing the epoxy components togetherand then slowly adding the polyetherimide or micronized PES viscositycontrol agents. The resulting mixture is heated to around 130° C. andmixed for a sufficient time to dissolve the polyetherimide/PESparticles. Once the polyetherimide/PES is dissolved, the mixture iscooled to around 75° C. The aromatic amine curing agent and the filletforming densified PES particles are then added to the mixture. The resinshould be kept at temperatures below about 70° C.-75° C. while thecurative agent and densified PES particles are being mixed into theresin. The final resin has a minimum viscosity of between 150 to 1500poise when said viscosity is measured by Rheometric Dynamic Analysis(Rheometrics RDA2) at settings of 2° C./min, 10 rads/sec and 0.8-1.0 mmgap. The preferred viscosity range is 300 to 1200 poise.

The finished resin is applied to the desired fabric to form the prepreg.The resin content of the prepreg may be varied depending upon a numberof different parameters in order to achieve desired mechanical andstructural properties for the sandwich panel. It is preferred that theprepreg have a resin content of 35-45 weight percent.

The prepreg is bonded to the edge-coated honeycomb core using vacuumand/or pressure and heated to cure the prepreg and polyamide and/orrubber-containing adhesive to form cured face sheets that are securelybonded to the honeycomb. The amount of vacuum, pressure and heatrequired to cure and bond the prepreg to the edge-coated honeycomb maybe varied depending upon the particular matrix resin formulation, theparticular polyamide and/or rubber-containing adhesive formulation, theamount of resin matrix in the prepreg and the amount of adhesive on thehoneycomb edge. In general, sufficient pressure must be applied to theprepreg to ensure that good contact is made between the honeycomb andprepreg face sheet to provide adequate bonding. When fillet-formingparticles are present in the matrix resin, the temperature and othercuring conditions are also selected such that the densified PESparticles are substantially dissolved during the curing/bonding process.

In accordance with the present invention, a polyamide and/orrubber-containing adhesive is applied to the edges of the honeycombprior to bonding with the composite face sheet. In one embodiment, theadhesive includes polyamide (nylon) as a major ingredient. These typesof adhesives are referred to herein as “nylon adhesives” or “polyamideadhesives” and include any of the known adhesive compounds that includenylon as a major component (at least 40 weight percent and preferably atleast 60 weight percent). Polyamide adhesives are available commerciallyand typically include polyamide in amounts ranging from 60 to 100 weightpercent. Within a polyamide adhesive, other components such as epoxiesmay also be blended, for example, in a concentration of 40-60 weightpercent of the total polyamide adhesive. Exemplary polyamide types thatmay be used include nylon 66, nylon 612, nylon 6. Preferred polyamideadhesives contain transparent thermoplastic polyamides that have glasstransition temperatures of from about 110° C. to 180° C. However,polyamide adhesives having glass transition temperatures in the range of30° C. to 180° C. may be used, if desired. Polyamide adhesives thatcontain only aliphatic blocks, such as ELVAMIDE, have glass transitiontemperatures that are at the lower end of the acceptable range (i.e. 30°C. to 60° C.). Polyamide adhesives that additionally containcycloaliphatic blocks and aromatic blocks, such as GRILAMID, have glasstransition temperatures that are at the higher end of the acceptablerange (i.e. 100° C. to 160° C.). Only those polyamide adhesives thatcontain aromatic blocks, such as GRILAMID TR55, have glass transitiontemperatures above about 160° C. Polyamide adhesives that containaromatic blocks are particularly preferred for use in high temperatureand high humidity environments. The Tg of the polyamide can be higherthan 160° C. if the percentage of aromatic blocks in TR55 is increased.

Polyamide adhesives are available commercially from E.I. DuPont deNemours & Company (Wilmington, Del.) under the trade names NYSOL(previously ZYTEL) and ELVAMIDE. NYSOL and ELVAMIDE are preferredpolyamide (nylon) adhesives for use as edge coatings. Other exemplarycommercially available polyamide adhesives are ORGASOL, which isavailable from Atofina Chemicals, Inc. (Philadelphia, Pa.) and GRILAMIDTR55 and TR90 that are available from EMS-Chime (Sumter, S.C.).Exemplary preferred polyamide adhesives include ELVAMIDE 8061, 8066 and8023R; and NYSOL 1020.

The commercially available polyamide adhesives described above aretypically supplied as a solid. The solid polyamide adhesive is dissolvedin a suitable solvent, such as a blend of water and alcohol, to form asolution that is applied to the honeycomb edge. The solvent isevaporated to leave a polyamide coating on the honeycomb edge. Ethanol,benzyl alcohol and combinations of ethanol and benzyl alcohol arepreferred solvents. The amount of solvent that is used to dissolve thepolyamide can be varied to provide a solution having the desiredviscosity for application to the honeycomb edge. In general, thesolutions of polyamide adhesive will include from 10 to 50 weightpercent polyamide.

ELVAMIDE 8061, 8066 and 8023R are exemplary adhesives. The ELVAMIDEadhesives are semi-transparent and contain only aliphatic blocks. Themelting points (ASTM D3418) of ELVAMIDE 8061, 8066 and 8032R are 156°C., 115° C. and 154° C., respectively. The relative viscosities are70-100 for ELVALMIDE 8061, 21-29 for ELVAMIDE 8066 AND 24-36 forELVAMIDE 8023R. The Shore Hardness (ASTM D2240) is 75 for ELVAMIDE 8061,72 for ELVAMIDE 8066 and 70 for ELVAMIDE 8023R. The tensile strength(ASTM D638) is 7,500 psi for ELVAMIDE 8061, 5,700 for ELVAMIDE 8066 and7,400 psi for ELVAMIDE 8023R. The elongation at break (ASTM D638) at 23°C. is 320% for ELVAMIDE 8061, 370% for ELVAMIDE 8066 and 370% forELVAMIDE 8023R.

GRILAMID TR55 and TR90 are transparent thermoplastic polyamides based onaliphatic, cycloaliphatic and aromatic blocks. TR90 contains aliphaticand cycloaliphatic blocks. TR55 contains aliphatic, cycloaliphatic andaromatic blocks. The glass transition temperature (ISO 11357) for TR55is 160° C. and 155° C. for TR90. The tensile E-modulus (ISO 527) forTR55 is 2200 MPa and 1600 Mpa. The tensile strength at yield (ISO 527)for TR55 is 9 MPa and 60 MPa for TR90. The elongation at yield (ISO 527)is 9% for TR55 and 6% for TR90. The tensile strength at break (ISO 527)is 50 MPa for TR55 and 45 MPa for TR90. The elongation at break (ISO527) is greater than 50% for TR55 and greater than 50% for TR90. Theball indentation hardness (ISO 2039-1) for TR55 is 120 MPa and 90 MPafor TR90. The dielectric strength (IEC 60243-1) is 31 kV/mm for TR55 and34 kV/mm for TR90. The comparative tracking index (IEC 60112) for TR55is 600 and 600 for TR90. The heat deflection temperature (ISO 75) at1.80 MPa is 130° C. for TR55 and 115° C. The heat deflection temperatureat 0.45 MPa is 145° C. for TR55 and 135° C. for TR90. GRILAMID TR55 is aparticularly preferred polyamide adhesive for use in accordance with thepresent invention.

The final honeycomb sandwich panel may be designed for use in hightemperature (above 160° F./71° C. wet and above) and/or high humidity(>80 percent relative humidity and above) environments. In thesesituations, it is preferred that polyamide be combined with one or morethermosetting resins to form polyamide adhesives that function well insuch high temperature and/or high humidity conditions. Epoxy, cyanateester, bismalemide and phenolic resins are exemplary of the types ofthermosetting resins that may be combined with polyamide to form hightemperature/high humidity polyamide adhesives in accordance with thepresent invention. Epoxy and phenolic resins are preferred. The resinsmay be used alone or in combination. Exemplary epoxies have beendescribed above. Exemplary phenolics include any of the commerciallyavailable phenolics, such as 23-983 Resin, which is available from DurezCorp. (Addison, Tex.), and 23056, available from Durez Corp. (Addison,Tex.). Preferred high temperature/high humidity polyamide adhesivesinclude polyamide in amounts ranging from 40 to 90 weight percentpolyamide and one or more thermosetting resins in amounts ranging from10 to 60 weight percent. Suitable curing agents for the resins need tobe included as is well know in the art. Coupling agents and otherconventional additives may be included as is also well known in the art.In addition, solvents for the thermosetting resins, such as toluene andxylene, may be used to form resin solutions that are combined withsolutions of polyamide (as described above) to form the hightemperature/high humidity polyamide adhesive solution. The amount ofthermosetting resins in such solutions will typically range from 10 to80 percent by weight.

Rubber in liquid or particulate form may be added to the polyamideadhesive. Polybutadiene rubbers are preferred withacrylonitrile-butadiene copolymer (NBR or Buna-N) being particularlypreferred. Any rubber may be used in combination with polyamide to formthe edge coating adhesive provided that it does not adversely affect theviscosity and adhesive properties of the adhesive. In addition toacrylonitrile-butadiene copolymer rubber, other exemplary rubbersinclude butadiene homopolymer rubber, styrene-butadiene (SBR or Buna-S)rubber, natural (gum) rubber, ethylene-propylene (EPR) rubber,carboxylated nitrile rubber and hydrogenated nitrile rubber. The amountof rubber that is added may be varied depending upon the particularpolyamide adhesive being used. Amounts of rubber on the order of 5 to 20weight percent of the total adhesive are preferred. It is also preferredthat the rubber that is incorporated into the polyamide adhesive is aliquid in order to prevent the viscosity of the polyamide adhesive frombeing too high. Solid rubber may also be used. However, it is preferredthat the solid rubber particles be pre-dissolved in a solvent, such asMEK, and then mixed into the polyamide solution. The addition of rubberimproves the room temperature toughness of the adhesive and reduces theadhesive performance at elevated temperatures. Accordingly, the amountof rubber that is added is selected to provide the desired degree ofincrease in room temperature toughness without reducing the hightemperature performance of the adhesive below desired levels.

In another embodiment of the present invention, rubber forms anessential ingredient of the adhesive with the inclusion of polyamidebeing optional. The rubber-based adhesive preferably contains athermosetting resin in combination with rubber. Such rubber-modifiedthermosetting resins typically contain a thermosetting resin as theprincipal ingredient with rubber being present in amounts that rangefrom 5 weight percent to 50 weight percent of the total adhesive. Any ofthe thermosetting resins mentioned above in connection with thepolyamide-thermosetting adhesive may be used. Such resins includeepoxies, cyanate esters, bismaleimides and phenolics. Epoxies arepreferred. However, it is also preferred that the type of thermosettingresin used in the rubber-modified adhesive be the same or similar to thethermosetting resin that is used as the resin matrix in the face sheetprepreg. For rubber-modified epoxy resins, the amount of rubber ispreferably between about 5 weight percent and 25 weight percent.Particularly preferred rubber amounts are on the order of 15 to 20weight percent.

Any of the various types of rubbers mentioned above can be combined withone or more thermosetting resins to provide an edge-coating adhesive inaccordance with the present invention. Acrylonitrile-butadiene rubber(NBR) is a preferred rubber. NBR, as well as the other types of rubberslisted above are available commercially from a number of sources.Examples include HYCAR CTBN which is available from Noveon SpecialtyChemicals (Cleveland, Ohio) and NIPOL NBR which is available from ZeonChemicals (Louisville, Ky.). NIPOL 1472, HYCAR 1300x13 and combinationsthereof are particularly preferred. HYCAR 1300x13 is a liquid rubberthat has an acrylontirile content of 26 percent. With respect to thecarboxyl content, the Acid Number is 32 and the Equivalents per HundredRubber is 0.057. The Brookfield Viscosity at 27° C. is 500,000 mPas. Theglass transition temperature (measured via Differential ScanningCalorimeter) is −39° C. and the functionality of the rubber is 1.8. Themolecular weight is 3150 Mn.

The rubber should have a molecular weight of between 2500 Mn and 60000Mn. The molecular weight(s) of the rubber are chosen to provide anadhesive that has a sufficiently high rubber content (at least 5 weightpercent and preferably 15-20 weight percent) while still having aviscosity that is low enough to allow it to be applied as an edgecoating in accordance with the present invention. The viscosity of therubber solutions and adhesive can be adjusted using solvent to achievedesired viscosity levels. The amount of rubber, solvent and otheringredients are selected to preferably provide a rubber-modifiedadhesive that has a viscosity of between about 1000 and 150000 poise at21° C.

The rubber-modified thermosetting resin may include any number ofadditives that are typically included in such resin systems. Exemplaryadditives include polyamide in amounts that are compatible with therubber in the adhesive. Any of the polyamides described in connectionwith the polyamide adhesive may be used. Other exemplary additivesinclude: flame retardants, such as tetrabromo bisphenol A; colorants,such as antimony trioxide; and anti-foam agents, which may be added intypical amounts for such additives.

The rubber-modified thermosetting resin should include a suitable curingagent, which may or may not be included as part of the thermosettingresin when it is mixed with the rubber component(s). Any of the variouscuring systems and agents may be used as described previously inconnection with the curing of thermosetting resins used in the facesheet prepregs. Tertiary amine catalysts, urea catalysts andcombinations thereof are preferred. Dicyandiamide and/or DDS arepreferred curing agents.

Rubber-modified thermosetting resins are available commercially from anumber of sources. However, these commercial resins typically do notcontain sufficient amounts of rubber to be suitable for use as an edgecoating adhesive in accordance with the present invention. It ispreferred that the rubber-modified thermosetting resin adhesive be madeby combining the thermosetting resin(s) with rubber in a multi-stageprocess. Solid rubber particles (e.g. NIPOL 1472) having a relativelyhigh molecular weight (typically 54000 Mn) are first mixed with asuitable solvent, such as methylethyl ketone (MEK) to form a rubbersolution in which the particles are completely dissolved. This rubbersolution is then added to a second solution that contains a mixture oflower molecular weight rubber (e.g. HYCAR 1300x13) and the thermosettingresin. Catalysts and other additives may be included in the secondsolution. The resulting mixture is heated to temperatures on the orderof 120° C. to 150° C. for a sufficient time (typically 60-120 minutes)to build up molecular weight and form a uniform solution. If desired,additional resin, rubber, additives and/or catalysts and the like can beadded to the two solutions after they have been mixed together andheated (i.e. pre-cooked). This type of staged addition of rubber, wheresolid rubber particles are first dissolved in a solvent and then addedto a liquid rubber/thermosetting resin mixture, is preferred in order toachieve relatively high rubber loading of the adhesive while keeping theviscosity low enough to provide adequate coating of the honeycomb edgeand improved face sheet bonding.

The polyamide and/or rubber-containing adhesive may be applied to thehoneycomb edge using any technique that provides a uniform edge coating.The preferred method is to dip or press the honeycomb edge into a layerof polyamide and/or rubber-containing adhesive solution. The adhesivelayer is preferably formed by pouring the polyamide and/orrubber-containing adhesive solution onto a flat surface and spreadingthe adhesive out using a Gardner's knife of other suitable blade. Theobjective is to form a uniformly thick layer that is from 0.001 inch(0.0254 mm) to 0.050 inch (1.3 mm) thick. The preferred range ofthickness for the adhesive layer is from 0.003 inch (0.08 mm) to 0.008inch (0.20 mm). The thickness of the layer may be increased or decreasedoutside this range, if desired, depending upon the thickness of thecore, cell size and wall thickness as well as the particular face sheetsbeing bonded to the honeycomb. The thickness of the adhesive solutionlayer determines the depth (see “D” in FIG. 3) of the coating on thehoneycomb edge. The preferred depth of the edge coating for any givenhoneycomb-face sheet combination may be determined by routineexperimentation involving varying the edge coating depth and measuringthe resulting bond (peel) strength of the honeycomb-face sheet bond.

The honeycomb edge is coated with adhesive by simply placing thehoneycomb edge onto the adhesive layer and moving the honeycomb downwardto contact the underlying surface. The honeycomb is then pulled awayfrom the underlying surface with adhesive solution remaining only on theportion of the honeycomb walls that was immersed in the adhesive layer.The solvent, if any, is evaporated to leave a coating of polyamideand/or rubber-containing adhesive on the honeycomb edge. Edge coating ofthe honeycomb in this manner is well know and preferred since itprovides a simple and effective method for applying a uniform coating ofadhesive to the edge of the honeycomb. Other methods involving powdercoating, spray, roller or brush application of the adhesive may also beused, if desired, especially for edge coating of relative largehoneycomb core.

The amount of polyamide and/or rubber-containing adhesive that isapplied to the honeycomb edge may be varied provided that the desireddegree of bond strength is obtained without adding too much weight tothe panel structure. The amount of polyamide and/or rubber-containingadhesive that provides the optimum combination of high bond strength andlight weight can also be determined by routine experimentation. This isachieved by not only varying the depth (D) of the edge-coating, but alsovarying the viscosity or tackiness of the polyamide and/orrubber-containing adhesive solution. Also, after evaporation of thesolvent, the core can be dipped again to pick up additional adhesive.This re-dipping process can be repeated as many times as necessary toachieve the desired level of adhesive loading. The drying temperaturedepends on the solvent used in the polyamide solution. For water-basedcoatings, the drying time is typically 4 minutes at 121° C. The amountof polyamide and/or rubber-containing adhesive applied to the honeycombedge is preferably between 1 g/sq. ft. (10.8 g/sqm) and 10 g/sq. ft.(107.6 g/sqm). However, the amount of adhesive that is used can bevaried widely depending upon the intended use for the honeycomb sandwichpanel and the desired combination of panel weight and face sheet peelstrength.

The bond formed by the polyamide and/or rubber-containing adhesive andface sheet resin matrix is preferably as strong or stronger than thestructural strength of the honeycomb. For example, it is preferred thatthe polyamide and/or rubber-containing adhesive, prepreg face sheet andhoneycomb are chosen such that (when the face sheet is forcibly pulledfrom the honeycomb) the body of the honeycomb (shown at 50 in FIG. 4)separates (see arrows 52) from the face sheet 53 prior to failure of thebond 54 between the honeycomb and face sheet bond. The use of apolyamide and/or rubber-containing adhesive edge coating in combinationwith composite face sheets in accordance with the present inventionprovides sufficiently high bond strength between the face sheet andhoneycomb so that it is routinely possible to make sandwich panels whereat least a portion of the core suffers structural failure prior to bondfailure during peel tests. The particularly high bond strengths providedby the present invention allows one to obtain maximum structuralstrength from a panel structure where the overall strength of the panelis limited by the structural strength of the core and not the bondbetween the face sheet and core.

Examples of practice are as follows:

COMPARATIVE EXAMPLE 1

A prepreg matrix resin was prepared having the following formulation:

23 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline);

25 weight percent GY281 (bis-F epoxy);

19 weight percent 3,3-Diaminodiphenylsulfone (3,3-DDS);

7 weight percent ULTEM® 1000P (polyetherimide); and

26 weight percent densified PES.

The densified PES was made from PES 5003P which is available fromSumitomo Chemical Co. Ltd. (Osaka, Japan). The PES was densified inaccordance with U.S. Pat. No. 4,945,154. MY0510 and GY281 were firstmixed in a mixing vessel, heated to 70° C. for approximately 10 minutes.The ULTEM® 1000P particles were then added and the resulting mixtureheated to 130° C. with mixing for approximately 75 minutes to fullydissolve the ULTEM® 1000P particles. The mixture was then cooled to 75°C. and the 3,3-DDS was mixed in for about 15 minutes. Then, thedensified PES was slowly added and mixed in for approximately 10 minutesto provide the final resin mixture. The viscosity of the homogeneousresin was measured over the entire curing temperature range (i.e., 20°C. to 177° C.) using Rheometric Dynamic Analysis as previouslydescribed. The resin had a minimum viscosity of 900 poise.

Face sheets were prepared by first forming a prepreg of 193 gsm 3K PWfabric containing 138 grams of resin per square meter. The prepreg wasformed as follows:

The resin was coated on release paper by reverse roller at about 175° F.(79° C.) to form a film containing 69 g/m². Two resin films wereimpregnated into the carbon fiber (in the form of a woven fabric) withan areal weight of 193 g/m².

The prepreg was applied to HRH® 10 core having ⅛ inch (0.31 cm) cellsand being ½ inch (1.27 cm) thick under vacuum at 22 inches (56 cm) Hgand cured for 2 hours at 177° C. with a pressure of 45 psi, venting at20 psi and ramp cooling at a rate of 2° C. per minute. The resultingspecimens were subjected to peel test according to ASTM D 1781. The facesheets all had peel strengths between 29 and 33 in-lb/3 in width.

COMPARATIVE EXAMPLE 2

A prepreg matrix resin was prepared in the same manner as ComparativeExample 1 except that the ingredients used to make the resin were asfollows:

21 parts by weight MY-0510;

21 parts by weight AcroCy® L-10;

21 parts by weight GY281;

9 parts by weight ULTEM® 1000P;

1.5 parts by weight CuAcAc/Nonylphenol (1/0.1); and

26.5 parts by weight densified PES.

The minimum viscosity of the homogeneous resin mixture was found to beabout 500 poise. The viscosity of the final resin mixture was measuredas set forth in Example 1. The final resin mixture was used to form aprepreg and applied to HRH® 10 core in the same manner as Example 1. Thepeel strength of the resulting face sheet was 26 in-lb/3 in width.

COMPARATIVE EXAMPLE 3

A prepreg matrix resin was prepared having the following formulation:

27.0 weight percent MY-0510 (N,N-Diglycidyl-4-glycidyloxyaniline);

24.9 weight percent GY285 (bis-F epoxy);

15.8 weight percent 3,3′-Diaminodiphenylsulfone;

1.3 weight percent Dicyandiamide;

13.5 weight percent micronized Polyethersulfone (PES); and

17.5 weight percent densified Polyethersulfone (PES).

Resin formulations may also be made wherein the amounts of MY-510, GY281and 3,3-DDS are varied by up to ±15%. Also, the amounts of both types ofPES may be varied by as much as +40%. The amount of dicyandiamide may bevaried by up to ±50%.

The densified PES was the same as used in Comparative Examples 1 and 2.Average particle size was 10-25 microns with no more than 13 weightpercent smaller than 5 microns and no more than 4 weight percent greaterthan 40 microns. 24.9 parts by weight of GY285 and 6.0 parts by weightof MY0510 were mixed in a resin kettle and heated, with stirring, to 65°C. Once this temperature is attained, 13.5 parts by weight micronizedPES 5003P is added to the resin kettle. The mixture is then heated to128±2° C. and held at this temperature for 75 minutes. At the end of 75minutes, heating is removed and 21 parts by weight of MY0510 are addedto the kettle. Stirring is continued as the mixture cools to 65° C. 15.8parts of 3,3-DDS is added and mixed for 15 minutes. 1.3 parts ofdicyandiamide is then added and the mixture stirred for 5 minutes at 65°C. Finally, 17.5 parts of densified PES is added and stirred in for 10minutes. The minimum viscosity of the resin was measured as set forth inExample 1 and found to be about 370 poise. Panels were prepared by firstforming a prepreg of 193 gsm 3K PW carbon fabric containing 70 grams ofresin per square meter. The prepreg was formed as follows:

The resin was coated on release paper by reverse-roll coater at about165° F. (74° C.) to form a film containing 70 g/m². The resin film wasimpregnated into a carbon fiber fabric having an areal weight of 193g/m². The prepreg was then applied to HRH® 10 core and cured in the samemanner as Comparative Example 1. The peel strength was around 32 in-lb/3in width on 3 pound core and around 31 in-lb/3 in width on 8 pound core.

COMPARATIVE EXAMPLE 4

Sandwich panels were prepared in the same manner as Comparative Example3 except that HRH®36, 2.5 pcf made with KEVLAR® was used as the coreinstead of HRH®10. The peel strength was found to be between 12 and 18in-lb/3 in width.

COMPARATIVE EXAMPLE 5

Sandwich panels were prepared in the same manner as Comparative Example4 except that the HRH®36 core was edge coated with various coatings thatdid not include a polyamide adhesive. One of the materials was A-22,which is a commercially available anionic surfactant that is availablefrom Cytec Industries, Inc. (West Paterson, N.J.). As described inUS00/5575882A, a significant increase in honeycomb modulus was observedafter immersing the core into a 2% by weight solution of A-22 anionicsurfactant. A second edge-coating material was 23056 which is a typicalphenolic resin for non-metallic core such as HRH®10 core. 23056 isavailable commercially from Durez Corp. (Addison, Tex.). A thirdmaterial was Phenolic coating 94-917, which is a modified phenolic thatis used as a coating for cans. Phenolic coating 94-917 is availablecommercially from Durez Corp. (Addison, Tex.). The honeycomb edges wherecoated with about 6.3 g/sq. ft. of each coating. All of the peelstrengths for the resulting panels were below 11 in-lbs/3 in width.

EXAMPLE 1

Sandwich panels are prepared in the same manner as Comparative Examples1 and 2, except that the edges of the honeycomb are coated with anaqueous solution of NYSOL 1020 (also know as ZYTEL FE310018) polyamideadhesive (40 weight percent polyamide) so that depth “D” of the adhesiveis between 0.005 inch (0.13 mm) and 0.008 inch (0.20 mm). The adhesive(coating) is applied to the honeycomb edges as follows: 1) a flat andlevel glass table top is cleaned with a degreasing solvent to remove alldebris and contaminants; 2) A Gardner's knife is set on the tabletop andthe gap setting at both ends of the knife are adjusted to provide thedesired film thickness; 3) The polyamide solution is poured onto thetable top parallel to the Gardner's knife and the knife is then draggedacross the adhesive with equal pressure on both ends of the knife toform a thin film that is slightly larger than the area of the core; 4)the resulting film is visually inspected to insure that a uniform filmof constant thickness has been formed; 5) the core is placed on top ofthe film and manually pressed downward with a consistent pressure toinsure uniform application of the adhesive film to the core; 6) the coreis then lifted from the table top and dried in an electric oven at 250°F. (121° C.) for 4 minutes; 7) the core is weighed to determine how muchnylon adhesive has been retained on the core edge before and aftercoating; and 8) the process is repeated, if necessary to obtain thedesired polyamide coating loading which in this example is between 3 and10 grams per square foot (g/sq. ft.). The panels prepared in thisexample will have peel strengths on the order of 90 in-lbs/3 in width.The adhesive bond between the core edges and the skins or face sheetsare sufficiently strong that the honeycomb wall, and not the core-skinbond, will structurally fail over at least a part of the honeycomb edgeduring the peel test.

EXAMPLE 2

Sandwich panels were made in the same manner as Example 1 except thatthe prepreg face sheets or skins were the same as those used inCOMPARATIVE EXAMPLE 3. The HRH®10 (8 pound) core was edge coated withthe NYSOL 1020 nylon adhesive solution to provide an adhesive loading of4.3 g/sq. ft. and the depth D of the coating on the core edge was about0.005 inch (0.13 mm). The peel strength of this panel was 91 in-lbs/3 inwidth with structural failure occurring in the honeycomb wall over asubstantial portion of the honeycomb and not at the core-skin bond.

EXAMPLE 3

Sandwich panels were made in the same manner as Example 2 except thatHRH®36 (2.5 pcf) core was substituted in place of HRH®10 core. The edgesof six different cores were coated with 1.1, 2.3, 3.4, 3.8, 4.0 and 6.2g/sq. ft. of polyamide adhesive (i.e., NYSOL 1020, 40% solids solutionin water, as per Example 1). The resulting panels had peel strengths of26, 30, 59, 71, 71 and 76 in-lbs/3 in width, respectively. During thepeel tests, the face sheets separated from the core at the bond betweenthe face sheet and the core. Based on this example, it is preferred forthis particular type of sandwich panel configuration that the edge ofthe honeycomb be coated with at least 3.0 g/sq. ft. of polyamideadhesive.

EXAMPLE 4

Sandwich panels were made in the same manner as Example 3 except thatELVAMIDE 8023R was used in place of NYSOL 1020 in the solution that wasapplied to the honeycomb. The solution was composed of: 15.9 weightpercent ELEVAMDE 8023R; 65.3 weight percent ethanol; 9.7 weight percentbenzyl alcohol; and 9.1 weight percent de-ionized water. The edges ofthree different cores were coated with sufficient solution to provideloadings of 1.8, 2.9 and 3.7 gm/sq. ft. of nylon adhesive. The resultingpanels had peel strengths of 37, 39 and 42 in-lbs/3 in width,respectively. During the peel tests, the face sheets separated from thecore at the bond between the face sheet and the core. Based on thisexample, it is preferred for this particular type of sandwich panelconfiguration that the edge of the honeycomb be coated with at least 1.0gm/sq. ft. of nylon adhesive this coating.

EXAMPLE 5

Sandwich panels were made in the same manner as Example 3 except thatthe prepreg face sheets were made using F593 Resin which is availablefrom Hexcel Corporation. The panels had peel strengths on the order of63 in-lbs/3 in width when the core edges contained NYSOL 1020 polyamidecoating weights of about 5.5 g/sq. ft. When the face sheets wereattached to the core without the polyamide edge coating, the peelstrength was only about 5 in-lbs/3 in width.

EXAMPLE 6

Sandwich panels are made in the same manner as Example 3 except that ahigh temperature/high humidity polyamide adhesive is used in place ofNYSOL 1020. The polyamide adhesive is preferably composed of ELVAMIDE8023R in combination with epoxy and phenolic resins along with asuitable amount of a curing agent and/or catalyst for the resins (e.g.4,4′-diaminodiphenylsulfone (4-4-DDS)) and a coupling agent, such asZ-6040 which is available from Dow-Corning (Midland, Mich.). In atypical formulation, a 25 to 30 weight percent solution of ELVAMIDE8023R (ethanol:benzyl alcohol—1:2 by weight) is first prepared. Thispolyamide solution is then combined with from 15 to 25 weight percent ofa novolak epoxy solution (e.g. 45 to 50 weight percent ECN 1299 epoxy intoluene/xylene) and 5 to 15 weight percent phenolic resin (e.g. 23-983Resin which is available from Durez Corp. (Addison, Tex.). 0.5 to 2weight percent of a curing agent (e.g. 4-4-DDS); 0.1 to 0.5 weightpercent of a phenylurea catalyst (e.g. DIURON available from E.I. DuPontde Nemours & Company (Wilmington, Del.); and 0.1 to 0.5 weight percentof a coupling agent (e.g. Z-6040) are also included. Other additives,such as AGERITE D Resin (RT Vanderbilt of Norwalk, Conn.) may be addedin small quantities (e.g. less than 1 weight percent).

The peel strengths of the sandwich panels made with the formulationabove with 2 to 4 g/sq. ft. (21.5 to 43 g/sqm) polyamide adhesiveloading, when measured at room temperature under dry conditions, willgenerally be about 32 in-lbs/3 in width. The peel strengths of the samesandwich panels will generally only drop to about 25 in-lbs/3 in widthwhen measured under wet conditions (e.g., 95% relative humidity orhigher) at an elevated temperature of 71° C. As nylon appears to be moresusceptible to water pick up than epoxy, with polyamide adhesivescomprising pure nylon, typical readings for peel strength are 37 to 42in-lbs/3 in width at room temperature (dry), and about 20 to 34 in-lbs/3in width at conditions of elevated hot/wet.

Panels made according to this example were also subjected to a flat-wisetensile test (FWT) according to ASTM C 297-94. The results of the FWT'sfor panels coated with 3.7 g/sq. ft and 5.2 g/sq. ft of thepolyamide/epoxy adhesive were 607 psi and 627 psi, respectively, at 93°C. and 438 psi and 401 psi, respectively at 135° C.

EXAMPLE 7

Sandwich panels were made in the same manner as Example 3 except thatELVAMIDE 8066 was used in place of NYSOL 1020 as the polyamide adhesive.The solution that was applied to the honeycomb was composed of: 16weight percent ELVAMIDE 8066; 38 weight percent ethanol; 23 weightpercent benzyl alcohol and 23 weight percent water. The edges of twodifferent cores were coated with sufficient solution to provide edgeloadings of 1.3 and 3.9 gm/sq. ft. of polyamide adhesive. The resultingpanels had peel strengths of 19.5 and 56.3 in-lbs/3 in width,respectively. Panels made with cores that were coated with 4.0 gm/sq.ft. of ELVAMIDE 8066 adhesive had FWT's of about 450 psi at 93° C. andabout 270 psi at 135° C.

EXAMPLE 8

Sandwich panels were made in the same manner as Example 3 except thatGRILAMID TR55 was used in place of NYSOL 1020 as the polyamide adhesive.The edges of two different cores were coated with 2.3 and 4.8 gm/sq. ft.of polyamide adhesive. The resulting panels had peel strengths at roomtemperature of 22.8 and 26.0 in-lbs/3 in width, respectively. The peelstrengths of the two panels increased to 24.6 and 28.6 in-lbs/3 inwidth, respectively, when the tests were conducted at 71° C. under wetconditions. Three additional panels were made with edge coatings of 4.1,5.2 and 5.8 gm/sq. ft. of GRILAMID TR55. The panels had peel strengthsof 24, 34 and 39 in-lbs/3 in width, respectively, at room temperature.The peel strengths of the panel increased to 25, 38 and 50 in-lbs/3 inwidth, respectively, at 93° C. The FWT for these three panels at 93° C.was 767 psi, 808 psi and 891 psi, respectively. At 135° C., the FWT forthe same panels decreased to 579 psi, 543 psi and 597 psi. Peelstrengths of the two panels (2.3 and 4.8 gm/sq. ft.) increased to 24.6and 28.6 in-lbs/3 in width, respectively, when the tests were conductedat 71° C. under wet conditions. As noted previously, GRILAMID TR55 isbased on aromatic blocks and has a relatively high glass transitiontemperature (Tg) when compared to other polyamide adhesives that containonly aliphatic and cycloaliphatic blocks. Accordingly, polymeradhesives, such as GRILAMID TR55, which have relatively high Tg's, arebetter suited for use at elevated temperatures and/or in a wetenvironment.

EXAMPLE 9

Panels were made in accordance with Example 8 except that GRILAMID TR90was substituted for GRILAMID TR55. The peel strength for panels havingcore edges coated with 5.2 gm/sq. ft. GRILAMID TR90 was about 23in-lbs/3 in width at room temperature and 27 in-lbs/3 in width at 93° C.The FWT at 93° C. was 639 psi. At 135° C. the FWT was 519 psi.

EXAMPLE 10

Panels were made in accordance with Example 8 except that mixtures ofELVAMIDE 8066 and GRILAMIDE TR55 were substituted for GRILAMID TR55.Three different mixtures of the two polyamides (Type A, Type B and TypeC) were prepared. In Type A, the polyamide mixture included 65 weightpercent ELVAMIDE 8066 and 35 weight percent GRILAMID TR55. In Type B,the polyamide mixture included 50 weight percent of each polyamideadhesive. In Type C, the polyamide mixture included 25 weight percentELVAMIDE 8066 and 75 weight percent GRILAMID TR55. Solutions containingthe Type A and Type B polyamide mixtures were coated onto the honeycombedge to provide adhesive weights of 4.0 gms/sq. ft. The peel strengthswere 34 and 42 in-lbs/3 in width, respectively, at room temperature. TheFWT at 93° C. was 459 psi for the Type A edge coating and 628 psi forType B. At 135° C., the FWT dropped to 292 psi for Type A and 344 psifor Type B. The Type B polyamide adhesive combination was also coated ata rate of 3.9 gms/sq. ft. As expected, the peel strengths were lowerthan the panels coated at a 4.0 gms/sq. ft. rate. However, the FWT at93° C. increased to about 660 psi. At 135° C., the FWT for the slightlylower loaded panel also increased to 391 psi.

The Type C combination was edge-coated onto the cores at a rate of 4.3gms/sq. ft. The peel strengths were about 30 in-lbs/3 in width at bothroom temperature and 93° C. The FWT at 93° C. was 659 psi. At 135° C.,the FWT dropped to 370 psi.

EXAMPLE 11

Panels were made in the same manner as Example 10, except that a smallamount of ECN 1299 epoxy was added to the ELVAMIDE 8066/GRILAMID TR55polyamide adhesive. Three different combinations of adhesive solutions(Type D, Type E and Type F) were prepared. The Type D adhesive solutioncontained 6.9 weight percent ELVAMIDE 8066, 6.9 weight percent GRILAMIDTR55, 1.2 weight percent ECN 1299, 83.1 weight percent benzyl alcoholand 1.8 weight percent acetone. The Type E adhesive solution contained2.9 weight percent ELVAMIDE 8066, 8.2 weight percent GRILAMID TR55, 1.0weight percent ECN 1299, 86.4 weight percent benzyl alcohol and 1.5weight percent acetone. The Type F adhesive contained 2.9 weight percentELVAMIDE 8066, 8.7 weight percent GRILAMID TR55, 0.61 weight percent ECN1299, 86.9 weight percent benzyl alcohol and 0.9 weight percent acetone.The peel strengths for all three of the adhesive types ranged from 28 to31 in-lbs/3 in width at both room temperature and 93° C. for coatingweights of about 4.2 g/sq. ft. The FWT for the Type D adhesive was 527psi at 93° C. and 365 psi at 135° C. The FWT for the Type E adhesive was700 psi at 93° C. and decreased to 456 psi at 135° C. The FWT for theType F adhesive was 696 psi at 93° C. and 442 psi at 135° C.

COMPARATIVE EXAMPLE 6

A sandwich panel was made in the same manner as Comparative Example 4except that the face sheet prepreg was made using an epoxy resin matrixthat is available from Hexcel Corporation under the tradename F155. Thehoneycomb was not edge coated and a separate structural adhesive was notapplied to the prepreg. The peel strength for the panel was 10 in-lbs/3in width at room temperature. The FWT was 579 psi at room temperatureand 501 psi at 71° C.

EXAMPLE 12

Sandwich panels were made in the same manner as Comparative Example 6except that the HRH-36 core was edge coated with a rubber-modified epoxyadhesive in accordance with the present invention. The adhesives wereprepared as follows. A rubber solution (Solution 1) was prepared bymixing 85 parts by weight MEK with 15 parts by weight NIPOL 1472. Themixture was kept at room temperature for 3 to 4 hours until the NIPOL1472 rubber particles had completely dissolved. A second solution wasprepared by mixing 302 grams of Solution 1 with 172 grams bisphenol A, 3grams of a tertiary amine catalyst, 136 grams of DER 331 epoxy resin,317 grams of GY 282 epoxy resin and 68 grams of HYCAR 1300x13 liquidrubber. All of these materials were mixed together and heated to 140° C.to 150° C. for 90 minutes to form Solution 2.

845 grams of Solution 2 was then mixed with 65 grams MEK, 32 grams ofGY2600 epoxy, 32 grams of pulverized dicyandiamide and 15 grams of aurea catalyst to provide a Solution 3. A first adhesive was prepared bymixing 300 grams of Solution 3 with 150 grams of a rubber solution thatcontained 20 weight percent NIPOL 1472 rubber particles dissolved inMEK. The total rubber content for this rubber-modified epoxy adhesive(RBE 1) was 16.6 weight percent. A second adhesive was prepared bymixing 265 grams of Solution 3 with 185 grams of the rubber solutionthat contained 20 weight percent NIPOL 1472. The total rubber contentfor this adhesive (RBE 2) was 20.4 weight percent.

The HRH-36 honeycomb was coated with 5.2 gms/sq. ft. of RBE 1 or 5.4gms/sq. ft. of RBE 2 prior to bonding of the face sheet to the core. Atroom temperature, the peel strengths for the panels were 41 and 52in-lbs/3 in width for RBE 1 edge coated cores and RBE 2 edge coatedcores, respectively. Corresponding peel strengths at 71° C. were 44 and50 in-lbs/3 in width, respectively. The FWT for RBE 1 coated cores atroom temperature and 71° C. were 708 psi (due to core failure) and 530psi, respectively. The FWT for RBE 2 coated cores at room temperatureand 71° C. were 693 psi (due to core failure) and 455 psi, respectively.

EXAMPLE 13

A sandwich panel was made in the same manner as Example 12 except thatthe HRH-36 core was coated with 4.0 gms/sq. ft. of the polyamide edgecoating of Example 4, except that ELVAMIDE 8066 was substituted forELVAMIDE 8023R. The peel strength was 47 in-lbs/3 in width at roomtemperature. The FWT at room temperature was 665 psi (due to corefailure) and the FWT at 71° C. dropped to 396.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above preferredembodiments, but is only limited by the following claims.

1. A method for bonding a face sheet to a honeycomb, said methodcomprising the steps of: providing a face sheet comprising an interiorsurface for bonding to said honeycomb and an exterior surface, said facesheet comprising at least one fiber layer and a resin matrix; providinga honeycomb having walls that define a plurality of honeycomb cellswherein said walls have at least one edge for bonding to the interiorsurface of said face sheet; applying an adhesive to said at least oneedge to form an edge-coated honeycomb wherein said adhesive comprises arubber-modified thermosetting resin; and bonding said face sheet to saidedge-coated honeycomb.
 2. A method for bonding a face sheet to ahoneycomb according to claim 1 wherein said rubber-modifiedthermosetting resin comprises a thermosetting resin selected from thegroup consisting of epoxy resin, phenolic resin, cyanate esters andbismaleimides.
 3. A method for bonding a face sheet to a honeycombaccording to claim 1 wherein said rubber-modified thermosetting resincomprises acrylonitrile-butadiene copolymer rubber.
 4. A method forbonding a face sheet to a honeycomb according to claim 2 wherein saidthermosetting resin is an epoxy.
 5. A method for bonding a face sheet toa honeycomb according to claim 4 wherein said rubber isacrylonitrile-butadiene copolymer rubber.
 6. A method for bonding a facesheet to a honeycomb according to claim 1 wherein said honeycombcomprises a composite material selected from the group consisting ofresin impregnated paper, resin impregnated glass fibers, and resinimpregnated carbon fibers.
 7. A method for bonding a face sheet to ahoneycomb according to claim 1 wherein said face sheet comprises aprepreg comprising a resin matrix.
 8. A method for bonding a face sheetto a honeycomb according to claim 7 wherein said prepreg isself-adhesive.
 9. A method for bonding a face sheet to a honeycombaccording to claim 8 wherein said resin matrix comprises a thermosettingresin.
 10. A method for bonding a face sheet to a honeycomb according toclaim 9 wherein said resin matrix comprises a thermoplastic resin.
 11. Amethod for bonding a face sheet to a honeycomb according to claim 1wherein said resin matrix is uncured and wherein said step of bondingsaid face sheet to said edge-coated honeycomb includes the additionalstep of curing said uncured resin matrix.
 12. A honeycomb sandwich panelcomprising: a face sheet comprising an interior surface that is attachedto said honeycomb and an exterior surface, said face sheet comprising atleast one fiber layer and a resin matrix; a honeycomb having walls thatdefine a plurality of honeycomb cells wherein said walls have at leastone edge that is attached to the interior surface of said face sheet;and an adhesive located at said one edge for forming a bond between saidface sheet and said honeycomb wherein said adhesive comprises arubber-modified thermosetting resin.
 13. A honeycomb sandwich panelaccording to claim 12 wherein said rubber-modified thermosetting resincomprises a thermosetting resin selected from the group consisting ofepoxy resin, phenolic resin, cyanate esters and bismaleimides.
 14. Ahoneycomb sandwich panel according to claim 12 wherein saidrubber-modified thermosetting resin comprises acrylonitrile-butadienecopolymer rubber.
 15. A honeycomb sandwich panel according to claim 13wherein said thermosetting resin is an epoxy.
 16. A honeycomb sandwichpanel according to claim 15 wherein said rubber isacrylonitrile-butadiene copolymer rubber.
 17. A honeycomb sandwich panelaccording to claim 12 wherein said face sheet is a prepreg.
 18. Ahoneycomb sandwich panel according to claim 17 wherein said prepreg isself-adhesive.
 19. A honeycomb sandwich panel according to claim 12wherein said resin matrix in said prepreg and said rubber-modifiedthermosetting resin have been cured to provide a cured honeycombsandwich panel wherein said face sheet is bonded to said honeycomb. 20.A honeycomb sandwich panel according to claim 12 wherein said honeycombcomprises a composite material selected from the group consisting ofresin impregnated paper, resin impregnated glass fibers and resinimpregnated carbon fibers.